The present invention relates to apparatus that provides relative movement between a light image and an area image sensor having sensor elements so arranged that each element samples a plurality of pixels of the light image.
Solid-state image sensors generally have a linear or area organization. A linear sensor will often have a row of sensor elements (usually photodiodes or photocapacitors) and one or more CCD shift registers. The elements sample a line of light from a light image and integrate (accumulate) charge representative of light passing through image pixels. After this integration, the charge is transferred to a CCD shift register. The charge is shifted out of the shift register and converted by an MOS transistor or diode into a voltage signal. This voltage signal is converted into a numerical representation suitable for input to a digital image processor. The next line of the image is then moved in front of the linear sensor and the above process is repeated. The integration time for an element has to be sufficiently long compared to the time for readout to minimize smearing. Linear sensors are inherently time-limited for image sampling. They are not satisfactory for use in some applications such as for example high speed printers for making prints of film negatives. Such a high speed printer requires an area image sensor that is capable of measuring the transmittance of a film negative quite rapidly, with high accuracy, and at high spatial frequency.
An area image sensor offers the advantage of increased integration time for each element. In some applications a large number of image pixels have to be digitized. For example, to make a high quality colored print of a photographic negative, on the order of about two million image pixels should be digitized for each color (red, green and blue) of a photographic negative to produce a high quality output print. With existing technology, area image sensors have about one hundred thousand elements. Thus each element of an area image sensor must sample a plurality of image pixels.
Interline area image sensors having a sensor with an array of sparse elements, provide an organization which lends itself to multiple element sampling of light image pixels. A sparse array of elements is one in which the elements are spaced from one another. The area image sensor is called interline since CCD shift registers are placed between columns of sensor elements. The light image need only be moved a relatively small distance for each sensor element to sample a different pixel of the light image. Digital images produced by sensors are stored in frame store memories. When digital images are produced by linear sensors or area sensors where a sensor element only samples a single light image pixel, then if a sensor element is defective it produces a defective digital image pixel. In one technique the value of a nearest neighbor digital image pixel is substituted. In other words, the procedure is simply to replace the value of a defective digital image pixel with the value of a nearest neighbor produced by another operative sensor element. In another technique interpolation of the values of nearest neighboring digital image pixels is used. See, for example, U.S. Pat. No. 4,253,120, issued Feb. 24, 1981, which teaches that, if lower resolving optics for imaging onto a high resolution imager are employed, any given image spot will overlap more than one pixel; and, attendantly, when a pixel is bad, interpolation can be employed to obscure, in real time, the effect of such bad pixel. However when an area image sensor having a sparse array of elements is used where each element senses a plurality of image pixels to produce a digital image, then some digital image pixels may not have nearest neighbors produced by other elements. In such a case the previous techniques for obviating the effect of one or more defective sensor elements may be less than acceptable.